Modern wireless communication devices typically employ a frequency synthesizer using phase locked loop technology to control the receive frequency of the device. A voltage controlled oscillator (VCO) provides a synthesizer output signal which is locked in frequency and phase to some multiple of a reference frequency provided by a reference oscillator. The synthesizer output signal is applied to a quadrature divider that processes the signal into an in-phase signal and, a quadrature signal, the quadrature signal being 90 degrees out of phase with respect to the in-phase signal.
The in-phase signal is applied as a local oscillator signal, ILO, to a mixer in the in-phase or I channel of the communication device. The quadrature signal is applied as a local oscillator signal, QLO, to another mixer in the quadrature or Q channel of the communication device. These mixers may be called the I channel mixer and the Q channel mixer, respectively. The receive signal from an antenna is supplied to both the I channel and Q channel mixers. Thus, the I channel and Q channel mixers mix the receive signal down to some intermediate frequency (IF). Other circuits couple to the I and Q channels to further process IF signals into a baseband signal. Finally, the baseband signal is processed to provide an audio signal that is supplied to an audio output of the communication device to which the user may listen.
The ILO and QLO signals cause the respective I and Q channel mixers to switch. It is desirable that the I and Q channel mixers still be able to switch despite the presence of large blocking signals along with the desired signal in the receive signal supplied to the mixers. Local oscillator (LO) swing is defined as the voltage excursion of the local oscillator signals, namely the ILO and QLO signals, supplied to the I and Q channel mixers. A large LO voltage excursion or swing is needed for low noise mixer performance. However, the voltage excursion of the LO swing can be so large that it overstresses or damages transistors within the I and Q channel mixers.
What is needed is a wireless communication method and device that provides a way to control LO swing to address the problems discussed above.